GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU CẤU TRÚC DỮ LIỆU DATA STRUCTURES [214331] Iterator - Comparable - Comparator /XX 12/3/15 Nguyễn Xuân Vinh nguyenxuanvinh@hcmuaf.edu.vn /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH Java Collection Architecture GV: NGUYỄN XUÂN VINH The Differences!!!  How to browse element?  Elements of List are indexed List index value 12 0 0 Linked List int value = list[0];  Elements of LinkedList, Sets and Maps can't be accessed MÔN: CẤU TRÚC DỮ LIỆU by index  How to remove element? "the" Set "from" /XX 12/3/15 Map "to" "we" GV: NGUYỄN XUÂN VINH Examining sets and maps  elements of Java Sets and Maps can't be accessed by index  must use a "foreach" loop: Set scores = new HashSet(); for (int score : scores) { System.out.println("The score is " + score); MÔN: CẤU TRÚC DỮ LIỆU }  Problem: foreach is read-only; cannot modify set while looping for (int score : scores) { if (score < 60) { // throws a ConcurrentModificationException scores.remove(score); } /XX 12/3/15 }  iterator: An object that allows a client to traverse the elements of any collection, regardless of its implementation  Remembers a position within a collection, and allows you to: get the element at that position advance to the next position (possibly) remove or change the element at that position MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH Iterators (11.1)  index value 12 0 0 size /XX 12/3/15 list Benefit: A common way to examine any collection's elements iterator set "to" "we" "from" current element: current index: "the" iterator current element: next element: "the" "from" GV: NGUYỄN XUÂN VINH Iterator methods hasNext() returns true if there are more elements to examine next() returns the next element from the collection (throws a NoSuchElementException if there are none left to examine) /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU remove() removes from the collection the last value returned by next() (throws IllegalStateException if you have not called next() yet)  Iterator interface in java.util  every collection has an iterator() method that returns an iterator over its elements Set set = new HashSet(); Iterator itr = set.iterator(); GV: NGUYỄN XUÂN VINH Iterator example Set scores = new HashSet(); scores.add(38); scores.add(94); scores.add(87); scores.add(43); scores.add(62); MÔN: CẤU TRÚC DỮ LIỆU Iterator itr = scores.iterator(); while (itr.hasNext()) { int score = itr.next(); System.out.println("The score is " + score); // eliminate any failing grades if (score < 60) { itr.remove(); 12/3/15 System.out.println(scores); } /XX } // [62, 94, 87] GV: NGUYỄN XUÂN VINH Iterator example Map scores = new HashMap(); scores.put("Kim", 38); scores.put("Lisa", 94); scores.put("Ryan", 87); scores.put("Morgan", 43); scores.put("Marisa", 62); MÔN: CẤU TRÚC DỮ LIỆU Iterator itr = scores.keySet().iterator(); while (itr.hasNext()) { String name = itr.next(); int score = scores.get(name); System.out.println(name + " got " + score); // eliminate any failing students if (score < 60) { /XX 12/3/15 itr.remove(); // removes name and score } } System.out.println(scores);// {Marisa=62, Lisa=94, Ryan=87} GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX Exercise  Modify the Book Search program from last lecture to eliminate any words that are plural or all-uppercase from the collection GV: NGUYỄN XUÂN VINH Set/Map and ordering  Some types have a notion of a natural ordering  TreeSet/Map store values sorted by their natural ordering Set scores = new HashSet(); scores.add(38); scores.add(94); MÔN: CẤU TRÚC DỮ LIỆU scores.add(87); scores.add(43); // unpredictable order scores.add(62); System.out.println(scores); // [62, 94, 43, 87, 38] Set scores = new TreeSet(); scores.add(38); scores.add(94); 12/3/15 System.out.println(scores); 10 scores.add(43); /XX scores.add(87); // sorted natural order scores.add(62); // [38, 43, 62, 87, 94] GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 21 Understanding Comparable public int compareTo(Object obj)  Elements must be mutually comparable:  You can only compare like elements  Or ClassCastException will be thrown  In most cases, the two objects must be of the same type (or a subtype) GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 22 Understanding Comparable  The return value states the relative position to the natural ordering:  compareTo() method can return one of three values: Negative number: current object > object compared to Positive number: current object < object compared to Zero: two objects are equal GV: NGUYỄN XUÂN VINH Understanding Comparable  The natural ordering should be consistent with equals():  Two elements are equal (equals()): compareTo() should return zero  Your class implements Comparable and inconsistent with equals(): won't work properly within a SortedSet or 23 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU SortedMap GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 24 Understanding Comparable  Never call the method directly:  Collections Framework call the compareTo() method for you when necessary  Not your responsibility to call the method 25 /XX Comparator 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 26 Comparator basics  Comparable objects: compare multiple instances of themselves  Comparator: compare two other objects  Custom ordering by creating a Comparator’s implement when:  Don't like the natural ordering  Your class doesn't implement Comparable GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 27 Comparator basics  Ordering is done through the compare () method GV: NGUYỄN XUÂN VINH Understanding Comparator  You have to implement compare()  compare()  compareTo()  But, both objects to compare must be passed in as arguments: 28 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU public int compare(Object obj1, Object obj2)  There is one predefined Comparator already implemented for you to sort elements in their reverse natural order:  Collections reverseOrder() comparator MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH Understanding Comparator When should we use Comparable or 29 /XX 12/3/15 Comparator? 30 /XX ITERABLE - ITERATOR 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 31 Iterator Object  An iterator is an object that implements the interface Iterator GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU Usage of Iterator  Provide a uniform way of accessing collection elements sequentially  in earlier versions of Java  From JDK 32 /XX 12/3/15 will work with anything that implements the interface Iterable GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 33 Directly implementing Iterable  Collection interface was made to extend Iterable  any collection can be the target of foreach  To build your own implementation of Iterable GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 34 Summary  Sorting: Comparable, Comparator  Iterable, Iterator 35 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH HỎI ĐÁP [...]... comparator MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH Understanding Comparator When should we use Comparable or 29 /XX 12/3/15 Comparator? 30 /XX ITERABLE - ITERATOR 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 31 Iterator Object  An iterator is an object that implements the interface Iterator GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU Usage... /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU SortedMap GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 24 Understanding Comparable  Never call the method directly:  Collections Framework call the compareTo() method for you when necessary  Not your responsibility to call the method 25 /XX Comparator 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX... into a random order sort(list) arranges elements into ascending order GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 19 Comparable basics  Ordering is done through the compareTo() method 20 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH System-Defined Comparable classes GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 21 Understanding Comparable public int compareTo(Object obj)... GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 33 Directly implementing Iterable  Collection interface was made to extend Iterable  any collection can be the target of foreach  To build your own implementation of Iterable GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 34 Summary  Sorting: Comparable, Comparator  Iterable, Iterator 35 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN... class doesn't implement Comparable GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 27 Comparator basics  Ordering is done through the compare () method GV: NGUYỄN XUÂN VINH Understanding Comparator  You have to implement compare()  compare()  compareTo()  But, both objects to compare must be passed in as arguments: 28 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU public int compare(Object obj1, Object... type (or a subtype) GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 22 Understanding Comparable  The return value states the relative position to the natural ordering:  compareTo() method can return one of three values: Negative number: current object > object compared to Positive number: current object < object compared to Zero: two objects are equal GV: NGUYỄN XUÂN VINH Understanding Comparable... thread "main" java.lang.ClassCastException at java.util.TreeMap.put(TreeMap.java: 542 ) 11 /XX 12/3/15 at java.util.TreeSet.add(TreeSet.java:238) at MyProgram.main(MyProgram.java: 24) GV: NGUYỄN XUÂN VINH Comparable (10.2) public interface Comparable { public int compareTo(E other); } 12 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU  A class can implement the Comparable interface to define a natural ordering... return 0; } 13 /XX } } // same x and same y GV: NGUYỄN XUÂN VINH compareTo tricks  subtraction trick - Subtracting related numeric values produces the right result for what you want compareTo to return: // sort by x and break ties by y public int compareTo(Point other) { if (x != other.x) { return x - other.x; // different x 14 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU } else { return y - other.y; // same x;...GV: NGUYỄN XUÂN VINH Ordering our own types  We cannot make a TreeSet or TreeMap of any arbitrary type, because Java doesn't know how to order the elements  The program compiles but crashes when we run it MÔN: CẤU TRÚC DỮ LIỆU Set tags = new TreeSet(); tags.add(new HtmlTag("body", true)); tags.add(new... - other.x < 0 == other.x, then x - other.x == 0 GV: NGUYỄN XUÂN VINH compareTo tricks 2  delegation trick - If your object's fields are comparable (such as strings), use their compareTo results to help you: // sort by employee name, e.g "Jim" < "Susan" public int compareTo(Employee other) { return name.compareTo(other.getName()); MÔN: CẤU TRÚC DỮ LIỆU }  toString trick - If your object's toString ... NGUYỄN XUÂN VINH GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 31 Iterator Object  An iterator is an object that implements the interface Iterator GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU... GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 19 Comparable basics  Ordering is done through the compareTo() method 20 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH System-Defined... Iterable GV: NGUYỄN XUÂN VINH MÔN: CẤU TRÚC DỮ LIỆU 12/3/15 /XX 34 Summary  Sorting: Comparable, Comparator  Iterable, Iterator 35 /XX 12/3/15 MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH HỎI ĐÁP